transformer question

ferronier · 18th March 2010, 10:08 PM

I am building a tube preamp from Rozenblit schematic; just got in the transformers, a Triad F4-120 and Triad F5-12. The spec sheets indicate 115v input; my mains are 122v. Output on the 120 is around 156v and output from the 12 is about 16v. Is this all because of the higher mains voltage? Should I lower the mains to get correct voltage, or lower the output? thanks

woody · 18th March 2010, 10:44 PM

There are 2 reasons for the higher voltage. If you place the transformer under a load
your voltages should come down to about the expected 120v and 12v. Small transformers like yours have to use a lot of verry fine wire so the load regulation isn't verry good. Try placing a load on that 12v winding to draw the rated current and you
will probably see that 16v drop to ~12v.

tomchr · 18th March 2010, 10:51 PM

I'm guessing you are measuring the transformer without load. The transformer should be rated at full load, so you will get a higher voltage if you measure without load.

The mains voltage will vary depending on a lot of factors. Time of day, time of year, your location relative to the substation, etc., etc. It looks like your mains voltage is within the 120 V +/-5 % spec. The voltage at my house measured 123.5 V the other night when I was testing my variac...

Bottom line: Don't worry about it.

~Tom

ferronier · 19th March 2010, 12:14 AM

Thanks guys, that makes sense. So, when my power supply is built, I can't really test it without a load, which in this case is supposed to be a tube...is there some dummy load you use to test transformers/power supplies? thanks

Ashley Ace · 19th March 2010, 02:37 PM

Dummy load is a bad choice. Dummy loads will have to draw the intended current and thus must be of any appropriate power rating. E.g. If you wish to test your 120V 0.2A (for example) tap under a load you`ll have to get a resistor with the parametres:

R = U/I = 120*5 = 600 Ohm, and
P = U*I = 120/5 = 24 W

To have some safety headroom, that will be a 30W resistor which is a rather BIG one.

18th March 2010, 10:08 PM	#1
ferronier diyAudio Member Join Date: Mar 2010	transformer question I am building a tube preamp from Rozenblit schematic; just got in the transformers, a Triad F4-120 and Triad F5-12. The spec sheets indicate 115v input; my mains are 122v. Output on the 120 is around 156v and output from the 12 is about 16v. Is this all because of the higher mains voltage? Should I lower the mains to get correct voltage, or lower the output? thanks

19th March 2010, 12:14 AM	#4
ferronier diyAudio Member Join Date: Mar 2010	(load)ed question Thanks guys, that makes sense. So, when my power supply is built, I can't really test it without a load, which in this case is supposed to be a tube...is there some dummy load you use to test transformers/power supplies? thanks

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18th March 2010, 10:44 PM	#2
woody diyAudio Member Join Date: Jan 2002 Location: Tyrone Ga. U.S.A.	There are 2 reasons for the higher voltage. If you place the transformer under a load your voltages should come down to about the expected 120v and 12v. Small transformers like yours have to use a lot of verry fine wire so the load regulation isn't verry good. Try placing a load on that 12v winding to draw the rated current and you will probably see that 16v drop to ~12v.

18th March 2010, 10:51 PM	#3
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	I'm guessing you are measuring the transformer without load. The transformer should be rated at full load, so you will get a higher voltage if you measure without load. The mains voltage will vary depending on a lot of factors. Time of day, time of year, your location relative to the substation, etc., etc. It looks like your mains voltage is within the 120 V +/-5 % spec. The voltage at my house measured 123.5 V the other night when I was testing my variac... Bottom line: Don't worry about it. ~Tom

19th March 2010, 02:37 PM	#5
Ashley Ace diyAudio Member Join Date: Mar 2010 Location: Moscow Region, Russia	Dummy load is a bad choice. Dummy loads will have to draw the intended current and thus must be of any appropriate power rating. E.g. If you wish to test your 120V 0.2A (for example) tap under a load you`ll have to get a resistor with the parametres: R = U/I = 1205 = 600 Ohm, and P = UI = 120/5 = 24 W To have some safety headroom, that will be a 30W resistor which is a rather BIG one.